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a� which case one probe per 1000 ft2 should be emplaced (California EPA, 2004). US EPA recommends several probes(e.g., 3 to 5; USEPA 2003). Prior to drilling holes in a slab, identify and mark utilities coming into the building from the outside (e.g., gas, water, sewer, electrical lines) and determine any internal locations where utilities penetrate the slab (e.g., furnace, water heater, circuit breaker box, water or sewer lines).Avoid installing sub-slab monitoring points where utilities penetrate the slab these may be potential entry points for downward oxygen migration through the slab. Also,avoid installing sub-slab points along straight-line points where utility trenches may have been installed beneath the slab during building construction. If available, consult any building construction records which may be available in sighting sub-slab monitoring points away from utility trenches. Prior to fabrication of sub-slab vapor probes, remove carpeting from the drilling location, if present. This can be done by cutting a small %2 inch square flap that can be glued back down after the probe is installed. Obtain any available information (e.g., from the owner, construction plans) to determine the thickness of the slab. Do not drill a pilot hole to assess the thickness of a slab. As illustrated in Figure 4, use a rotary hammer drill to create a "shallow" (e.g., 2.5 cm or 1 in deep) "outer" hole (e.g., 2.2 cm or 7/8 in diameter)that partially penetrates the slab. Do not completely penetrate the slab with the shallow hole. Use a small portable vacuum cleaner to remove cuttings from the hole. Removal of cuttings in this manner in a non-penetrated slab will not compromise soil vapor samples because of lack of pneumatic communication between sub-slab material and the vacuum cleaner. Next, use the rotary hammer drill to create a smaller diameter "inner" hole (e.g., 0.8 cm or 5/16 in diameter) through the remainder of the slab and some depth (e.g., 7 to 8 cm or 3 in) into sub-slab material. Figure 5 illustrates the appearance of"inner" and "outer" holes. Drilling into sub-slab material will create an open cavity which will prevent obstruction of probes by small pieces of gravel. The basic design of a sub-slab vapor probe is illustrated in Figures 5 and 6. Once the thickness of the slab is known, stainless steel or brass tubing should be cut to ensure that the probe tubing does not reach the bottom of the hole (to avoid obstruction of the probe with sub-slab material). Construct sub-slab vapor probes from small diameter (e.g., 0.64 cm or 1/4 in outer diameter (OD) x 0.46 cm or 0.18 in inner diameter (ID)) chromatography grade 316 stainless steel or brass tubing and stainless-steel or brass compression to thread fittings (e.g., 0.64 cm or 1/4 in OD x 0.32 cm or 1/8 in (ID) Swagelok® or NPT female thread connectors) as illustrated in Figure 6. Use stainless- steel or brass materials to ensure that construction materials are not a source of VOCs. Set the sub-slab vapor probe in the hole. As illustrated in Figure 7, the top of the probe should be completed flush with the slab and have recessed stainless steel or brass plugs so as not interfere with day-to-day use of the building. Mix a quick-drying Portland cement with water (which expands upon drying to ensure a tight seal) to form a slurry and inject or push into the annular space between the probe and outside of the "outer" ETC Soil Vapor Sampling Technical Toolkit 12 Version 1.4, September 7,2005